These experiments propose to define the separate distribution of sympathetic and parasympathetic nerve projections onto the heart, with specific proposals designed to selectively denervate sinoatrial (SAN) and atrioventricular (AVN) nodal regions, and prepare conscious canine models for study of the particular influences of each limb of the autonomic nervous system (ANS) upon these critically important regions of the heart. With such models we shall study reflex behavior in the presence of predetermined ANS imbalanced input to the heart. The study proposed (1) to prepare selectively denervated heart models and then (2) to study cardiovascular reflex function in the awake, normally functioning animal sustaining selective denervation (sympathectomy, parasympathectomy, or both) of the SAN or AVN regions of the heart. Animals with total denervation of the AVN region, innervation of SAN region remaining essentially intact, will be prepared. We shall also sympathectomize (parasympathetics remaining essentially intact), or parasympathectomize (sympathetics remaining intact) the AVN region while the SAN region retains a major portion of its innervation. Comparable models will be prepared in which total, sympathetic, or parasympathetic innervation to the SAN region are ablated while innervation of the AVN region remains essentially intact. The application describes a surgical procedure for total cardiac denervation from an intrapericardial approach which leaves the innervation of other thoracic and abdoninal structures intact. The procedure lends itself to selective extirpation of neural projections to highly restricted portions of the heart. Electrical stimulation of proximal portions of the ANS while recording inotropic, chronotropic and dromotropic events in the heart permit accurate evaluation of remaining innervation and/or selective ablations, before and after each stage of the surgical procedure. Once surgical procedures necessary to accomplish selective denervation are established, conscious dog models will be tested at rest, during exercise, and during stresses designed to test functional operation of those adrenergic and cholinergic mechanisms remaining intact. Performance of structures deprived of normally "balanced" inputs from ANS, as well as those sustaining "hypersensitivity" will be examined.